In various fields, delicate or valuable articles must be safely transported between work stations and the like without damaging or destroying the articles. Articles requiring careful handling include, but are not limited to, pharmaceuticals, medical systems, flat panel displays, computer hardware as for example disc drive systems, modems and the like, and semiconductor wafers.
In general, integrated circuits are manufactured by forming a plurality of layers on a substrate such as a semiconductor wafer. A variety of processing machines are used to form the individual layer, with the wafer typically being delivered to several different machines before the integrated circuits are completed. In addition to equipment for depositing films on the wafer, the semiconductor wafer may also be cleaned or conditioned at various stages by suitable equipment. With advances in technology, integrated circuits have become increasingly complex and typically include multiple layers of intricate wiring. The size of the integrated circuits has decreased, greatly increasing the number of such devices on a single wafer. As a result of the increased complexity and decreased size of the integrated circuits, the value of the semiconductor wafer increases substantially as the wafer progresses through the various processing stages. The standard size of the semiconductor wafers will increase from 200 mm to 300 mm in the next few years, further increasing the number of integrated circuits which may be formed on a single wafer and therefore the value of each wafer. Considerable care must be taken in handling the semiconductor wafers, particularly during the later processing stages, since a damaged wafer could result in considerable monetary losses. The semiconductor wafers must be retained in a clean room environment, substantially free of particulate contamination, to preserve the purity of the layers deposited on the wafer. The requirement of a clean room environment places additional constraints on the handling of the semiconductor wafers.
For additional protection against contaminants, the semiconductor wafers are typically retained in sealed transport pods as they are moved throughout the manufacturing facility to minimize any exposure to the environment outside of the processing machines. The manufacturing facility is usually organized into a plurality of bays each including several processing machines. After the wafers in a pod have been treated at one or more of the machines, the pod leaves the bay and is transported to the next processing bay. Thus, there are essentially two types of transport loops in the manufacturing facility--the inter-bay loop in which the pods are moved between the bays, and the intra-bay loop in which the pods are moved between the processing machines of a single bay. A system which may be used to conveniently, safely and efficiently handle and transport the pods is desirable. A pod transport system which maximizes the utilization of the machines in the processing bay is also desirable.
Various conveying systems have been employed to transport the pods from bay to bay along the inter-bay loop of a manufacturing facility. Because of the amount of traffic in the inter-bay loop of the manufacturing facility, inter-bay transport is typically accomplished via overhead conveyor systems. The pods are delivered to a robotic storage house, often referred to as a "stocker", which receives the pods and automatically delivers the pods to the intra-bay loop. With some systems, the inter-bay conveyor system is coupled to the intra-bay conveyor system for direct transfer between the systems. However, direct transfer may be obtained only when a compatible, overhead conveyor system is used in the intra-bay loop.
Within the bays, the transport pods must be carried from machine to machine and delivered to a position where the wafers may be unloaded from the pod by the machine for processing. The machine entrance is often provided with a load port where the wafers may be automatically removed from the transport pod in a protected environment. Transferring the pods to the load port requires greater precision and control over the pod than moving the pods between the inter-bay conveyor and the bays. Various methods are employed to move the transport pods between the different processing machines in a bay. For example, many systems rely upon human workers to transfer the transport pods from port to port using a cart. The worker may manually lift the pod to the port. Alternatively, the worker may actuate a manual robotic link or other lifting device to move the pod to the port and, after processing has been completed, to return the transport pod to the cart. The worker then moves the cart to the next machine and repeats the process. Relying on human workers to transport the pods from machine to machine is time consuming and inefficient. Often, the worker will not be on hand to position a pod of fresh wafers in the load port and the machine will sit in a stand-by mode reducing the time during which the machine is operating and the overall efficiency of the processing factory. Moreover, care must be taken to ensure the lifting device is properly aligned with the load port as dropping the pod or exposing the pod to sharp jolts may damage the wafers and could cause up to millions of dollars of damage. A means of automatically moving the transport pods between machines is desirable.
Another system of intra-bay transport relies upon automatic guided vehicles (AGVs) which carry the pods between the machines and move the pods into the load port. Using AGVs reduces the need for a worker in the bay and may increase the speed at which the pods are moved through the bay. However, the size of the bay limits the number of AGVs which may operate in a single bay, leaving the machines in a stand-by mode waiting for the AGV to remove the pod of processed wafers and deposit a pod of fresh wafers in the transfer bay. An automated system which may be used to rapidly deliver pods to and remove pods from the processing machines without leaving the machines in a stand-by mode is desirable.
Overhead monorail systems are also used to transport pods along the intra-bay loop. Hoists or similar devices are used to lower the pods onto the load port of the processing machine. In order to successfully transfer the pod from the monorail to the machine, the pod must be precisely aligned with the load port and lowered onto the port in a controlled manner such that any swing of the pod is minimized. After processing, the pod is raised and transported to the next machine. Repeatedly raising and lowering the pod is challenging. An automated conveyor system which positions the pod for direct, efficient transfer to the load port is desirable.
Conveyor systems for transporting materials are well known. Examples of standard conveyors include conveyor belt systems and roller systems where the articles are transported across a plurality of rotating rollers. While these systems provide a useful means of transport in most circumstances, they are not suitable for transporting pods in a clean room environment. Moreover, these systems do not offer precise control over the acceleration and deceleration of the pod which is required to prevent shifting of the wafers within the pods.
Another type of conveyor system which may be adapted for clean room use includes a pair of spaced rails each having a drive system for supporting an article and propelling the article along the rails. The competition between the two drive systems may cause the article to shimmy as it moves along the rails. A modification of this conveyor system includes a drive system on one rail and guide wheels on the other rail to allow the article to move freely along the rails. Unless the drive system, guide wheels and features of the transport pod are in exact, horizontal alignment, the guide wheels may cause the article to tip slightly such that each guide wheel imparts a slight impact on the article. While these adverse effects may be a minor inconvenience for most articles, the vibrations can have adverse effects on the delicate, expensive semiconductor wafers carried by the transport pod. A conveyor system for safely and protectively transporting semiconductor wafers is desirable.